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Alignment ideas and modifications

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Blog entry by Mike_190930 posted 10-31-2019 08:04 PM 263 reads 1 time favorited 3 comments Add to Favorites Watch

For the past couple of months, on and off, I’ve been constructing the Woodsmith CNC described in Issues 242/243. I view this as a first exploration into home-built CNC machines, so it is both a learning experience and hopefully, at the end, a very useful addition to the shop. Bruce MacDonald’s build thread on this forum has been inspirational, so I refer you to his postings for the bigger picture of the project, his modifications, and especially his ‘lessons learned’ post within that thread. In this blog, I will focus just on a few modifications I have made to the build and the reasons why. This is not to take anything away from either the Woodsmith plans or Bruce’s thoughtful and beautifully done build, but rather to make some aspects of the build a bit less demanding to construct, and perhaps even improve the precision of the CNC.

I’m generally not very good at hitting everything “dead nuts” or to within a 1/64’th inch, but doing otherwise would leave a lot to be desired for the CNC machine accuracy. So I came up with a few work-arounds that some of you may find useful. Here is a list of the critical alignments I wanted to hit. For reference, the x-axis is the long dimension of the bed.

1. The gantry rails should be perfectly parallel to the table (the left and right sides should be at exactly the same height above the table bed).
2. Compensate for any small twist (around the y axis) in the gantry bridge because of misalignments in the gantry supports. The router spindle should remain perfectly perpendicular to the bed over the entire y-axis travel range.
3. Square the z-axis rails to the bed in the x direction.
4. Square the z-axis rails to the bed in the y direction.
5. Ensure the router spindle axis is perfectly parallel to the z-rails. If not, the router bit will cut into the stock a little ‘sideways’ during a plunge.
6. Square the gantry y-axis rails to the x-axis rails.

That’s quite a list, but it turns out some simple zero-cost modifications to the basic Woodsmith design can build in adjustments for all of these. Here is how I made each of these critical alignments easy to adjust once the basic mechanics of the CNC are assembled. Remember though, these modifications can only compensate for just so much off-tolerance in the initial build, so don’t abandon good construction practices and do try to hit the 1/64” tolerances as close as possible. Use these modifications to tweak out that last annoying bit of misalignment, and to compensate for wear during use later on. Remember, any amount of off-square in the setup of the CNC will translate into each and every project you cut on it.


o Regarding alignments 1, 2, and 3 above: I modified both upper v-groove roller bearing axels on each gantry side to have the same offset dowel cam mechanism as the original lower two axels. Keep the same centers, but bore out the upper axels to 1” diameter as was done for the lower two. This allows about 1/4” of height adjustment on each side so you can make the gantry height exactly the same at both sides. As a bonus, you can now easily square the vertical gantry face to the table by adjusting the front axle cams on each gantry side. Finally, you can tweak out any twist that might have crept in between the two gantry sides by adjusting the top two axels on one side. Yes, it takes some patience to get all three of these things on the money, but at least you now have to facility to do so without re-making one part or another. It took me a couple of hours with wrenches and a square to get it all lined up properly, so it’s not really that bad. Here’s what it looks like.

o To adjust the z-axis rails square to the bed (point 4 above) in the y-axis direction: Add a third axel cam to the gantry runner plate (parts HH and II). This allows approximately +/- 1 degree tilt on the y-axis direction, allowing you to get the z-rails perfectly perpendicular to the bed. Again, just bore out the top right axel hole to 1” diameter. For this, either cut the third scallop on part II larger before gluing it to HH, or use a plunge router to enlarge the existing scallop. By the way, have you discovered the inconsistent dimensions shown between parts II and HH? Regardless of whether you use this mod, you’ll have to correct the dimensions on HH. The cutout centers in HH must have the same center to center distance as the axel centers on II, and also the same bottom edge to center dimension.

Here’s a photo of that modification.

o To adjust the spindle parallel to the z-axis rails (point 5 above): Add a third axel cam to the router carrier plate (part “OO”) at its upper right corner. This will allow you to dial the router spindle parallel to the z-rails (and perpendicular to the bed in the y-axis direction). This modification allows about +/- 1 degree of tilt along the y-axis direction relative to the bridge runner plate (part HH). Another photo:

So, with the addition of 6 additional cam points, you can tweak out misalignments for everything on the list above except number 6. Note that none of these address any out of square issues between the x- and y- axes. That you will have to address that during the initial construction of the gantry. The carriage plate for the gantry, part “W”, must be aligned carefully with the gantry sides during construction in order to get x and y exactly perpendicular. If any of you figure out a way to tweak this after construction, please post it here.

Now, there is one last thing to check. If the bed itself is not exactly flat, you will get some error in the z-height one place or another. For the truly exacting builder, you can mount a dial gauge in place of the router and sample a few dozen points around the bed. If that is an issue you cannot live with, chuck a dish carving bit into the router, and have the CNC “paint” the bed, taking off 1/64” or whatever you need in order to get the bed trued up relative to the side rails and gantry. Do this with the t-slot boards removed, and be sure that all screw heads are below the intended depth, or the router bit will hit them. Or, you could do this to true up a waste board every time you change it.

Hope this helps some of you who are building or contemplating building the Woodsmith CNC. Let me know how it works out.

-- Whadaya mean it ain't "measure once cut twice"?



3 comments so far

View Bruce Macdonald's profile

Bruce Macdonald

34 posts in 3666 days


#1 posted 11-06-2019 06:00 PM

Hi Mike,
Great comments. I like the idea of adding in the extra cam bearings – the only downside being you are starting with 4 unknown locations instead of 2 but I can certainly see the benefits and it likely isn’t as big a deal to dial it in perfectly. Nice to be able to get one direction perfectly square then move on to the next and then the next. Certainly has merit.
Yes I did notice (and corrected) the dif between II and HH – indeed they are incorrect.

I built a spoil board for mine with Tee nuts in the back on a 2” x 2” grid. Once installed I used a Whiteside flat planing bit in the router to create a perfectly flat surface (to the router). I then carved in a 2” x 2” grid. Works really well.

I just today finally got dust collection working. I purchased a dust shoe made specifically for the Dewalt router and tied it into my primary dust collection. You still get some extraneous bits flying around but nowhere near as bad as it was trying to follow the bit with a shop vac! LOL

The software I bought is Vectric VCarve Desktop v10.0 – Fantastic stuff it really works well. Now spending a bunch on tooling… never ends.

I’d be really interested to see more pics of your build. Did you stay with the anti-backlash nuts or did you go with ballscrews? Even Chris Fitch (author at Woodsmith) told me that he was seeing a bit of slop using the nuts only.

Anyways looks like you are well on your way – like I said please continue to post pictures.
I’ve suggested to Chris that they run a follow-up article on all those who built the CNC to see the variances and results achieved by those who undertook it. I’ll post in here if they go with it.
Bruce

View Mike_190930's profile

Mike_190930

17 posts in 43 days


#2 posted 11-06-2019 11:07 PM

Thanks for the comments Bruce. Had to take a break from this build for a couple of weeks while I read up on the motor/electronics end of it and also dealt with some significant car repairs. For me, the “wood” side of the built is essentially finished; now to get the electrical parts on it. Pictures to follow as that gets going.

For now I kept the backlash nuts, but just installing them I do see the probabliity that there will still be some unwanted movement. I do like the ball screws, but had already bought the acme parts before you posted. It does look like the CNC can be refitted later with ball screws without too much pain (outside of the cost). I’ll run it with the acme screws first and see how good/bad it is relative to my usage.

I once flattened a large walnut slab with a router sled; my gosh that thing piled up the sawdust. I was literally shoveling sawdust into a barrel after each pass. Dust collection is at the very top of my list! My CNC is fitted, for now, with a DeWalt DWP611 1.25 hp router. I’ll look into getting that bespoke dust shoe.

This week, I plan to install the Vectric Software and learn it while waiting for the next round of parts deliveries.

One feature I forgot to add in the original posting: If you don’t like the added cams on the axles, it is simple to revert to a fixed position axle again. Just make a 1” dowel insert that is center drilled for the axle.

-- Whadaya mean it ain't "measure once cut twice"?

View Mike_190930's profile

Mike_190930

17 posts in 43 days


#3 posted 11-06-2019 11:54 PM


– the only downside being you are starting with 4 unknown locations instead of 2 but I can certainly see the benefits and it likely isn t as big a deal to dial it in perfectly.

- Bruce Macdonald

I used a Wixey angle gauge to set the initial angles of the two gantry supports. The gauge is first zeroed to the bed. The two gantry attachment blocks are what count.

Before adjustment:

After adjustment:

At this point, I also set the distance from gantry support block to the bed to be the same on both sides by adjusting the top two cams on one of the gantry supports. The actual distance doesn’t much matter so long as they are both the same.

Next, I dry fit the gantry box, as well as the drive plate underneath. An array of bar clamps held everything in position. The two levels in the photo were used only as straight bars since they happened to be handy. I did not use them for level or plumb references. The cross piece of plywood laying on the bed is used to reference the square and thus to the edge of the bed, making the x and y axes perpendicular to each other. This assumes the remainder of the stuff mounted to the gantry face will keep the y-rails perfectly parallel to the position established at this step.

A little fine tuning was needed later. These two photos show the method I used to fine tune the z-axis rails to the bed in both the x and y tilt directions. I actually set up the rails before adding the router carrier plate, but forgot to grab a photo – but you get the idea from these.

Finally, I ran the gantry drive plate from left to right, checking that it was perpendicular to the long (x-axis) direction of the bed. A very slight adjustment of the two upper axles on one side support fixed a small twist in the gantry, and a quick double check on height showed I had managed not to disturb that measurement.

With everything now adjusted and still clamped in place, i screwed the gantry box to the supports, and the drive plate underneat was screwed to the gantry sides.

I am pleasantly suprised at how easily the gantry rolls down the bed. Same can be said for the y-axis and z-axis rollers.

-- Whadaya mean it ain't "measure once cut twice"?

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